1. The Field of the Invention
The present invention is related to methods and compositions for modifying the burn rate of solid rocket motor propellants, without the addition of expensive, toxic or polluting materials, such as lead or copper. More particularly, the present invention is related to the use of bismuth trioxide to modify the burn rate of solid rocket motor propellants.
2. Technical Background
In the manufacture of solid rocket motors, several components have been found to be required. First there must be an adequate rocket motor case. The rocket motor case forms the exterior of the rocket motor and provides the essential structural integrity for the rocket motor. The rocket motor case is conventionally manufactured from a rigid, yet durable, material such as steel or filament wound composite.
Placed within the interior of the rocket motor case is the propellant grain. The propellant forming the grain is conventionally burned to form thrust within the interior of the rocket motor case. The formation of hot gases upon burning of the propellant, and the subsequent exit of those gases through the throat and nozzle of the case provide the thrust to propel the rocket motor.
There are two major classes of propellants used in conventional applications. These include solid propellants and liquid propellants. Solid propellants are used extensively in the aerospace industry. Solid propellants have developed as the preferred method of powering most missiles and rockets for military, commercial, and space applications.
Solid rocket motor propellants have become widely accepted because of the fact that they are relatively simple to manufacture and use, and they have excellent performance characteristics. Furthermore, solid propellant rocket motors are generally more simple than liquid fuel rocket motors. For all of these reasons, it is found that solid rocket propellants are very reliable and economical.
In some applications, it is important that the rocket motor perform with reduced or eliminated smoke output. For example, in tactical rocket motors, the production of smoke causes a number of disadvantages. The smoke produced may obscure the vision of pilots or drivers of a craft or vehicle firing the tactical rocket. In addition, the production of smoke makes tracking the source of the motor easier, a serious disadvantage during military operations.
An important consideration in solid propellants, including minimum smoke propellants, is means for controlling the burn rate of the propellant, without significantly adding to the smoke output of the propellant. It will be appreciated that it is important that the propellant burn at a controlled and predictable rate. If the burn rate of the propellant can be controlled it is possible to assure proper operation of the rocket motor, or other similar device.
If the propellant achieves an excessively high burn rate, the pressure created within the casing may exceed the design capability of the casing, resulting in damage or destruction to the device. If the propellant does not develop a sufficient burn rate, there may not be sufficient thrust to propellant the rocket motor over the desired course.
Accordingly, it is conventional in the art to add materials to the propellant to control the burn rate of the propellant. Such materials are often referred to as burn rate modifiers. Burn rate modifiers are generally added in order to cause the burn rate of the propellant to "plateau" at an operable level. When burn rate is plotted as a graph of burn rate (for example, in inches per second) on the Y-axis and pressure (in pound per square inches) on the X-axis, the plateau effect results in a flatting of the burn rate curve to a slope more parallel with the X-axis. This plateau effect is desirable in order to achieve a relatively constant pressure output over a chosen period of time.
In order to achieve the plateau effect described above, it has been common practice to add relatively hazardous metals to the propellant. For example, lead is perhaps the most widely used burn rate modifier for certain classes of propellants. Lead, however, is known to be a hazardous, toxic, and polluting metal. Concern with lead pollution in society as a whole is on the rise, and serous health problems are known to be associated with lead poisoning and lead pollution. As a result, concern with lead in the preparation and use of propellants is high, and it is presently preferred that lead be eliminated as a component of solid propellants.
Another significant concern in the formulation of propellants is safety. Because of the fact that propellants are often used in environments where accidental ignition of the propellant is a possibility, preventing accidental ignition is of interest. If a propellant is capable of ignition by being struck by stray bullets or flying debris, the safety hazard is significantly increased. If the propellant will ignite at temperatures typically encountered in hot environments or under normal operating conditions, the usefulness of the propellant is significantly decreased. Thus, it is an object of propellant development to produce effective propellants which are also relatively insensitive to accidental ignition.
Accordingly, it would be a significant advancement in the art to provide methods and compositions for modifying propellant burn rates which avoided some of the significant problems encountered with conventional burn rate modifiers. More particularly, it would be an advancement in the art to provide burn rate modifiers which are not based on lead, copper, or similar materials. Similarly, it would be a related advancement in the art to provide methods and compositions for modifying burn rates in propellants which did not rely on toxic, hazardous, or polluting burn rate additives. It would be a further advancement in the art to provide such propellants which produced a minimum of smoke output when burned. It would be another advancement in the art to provide propellant compositions which are generally insensitive.
Such methods and compositions are disclosed and claimed herein.